Feeding human babies with non species specific, bastardised milk has a huge negative impact but the manufacturers have done a sterling job in convincing us that it’s awesome stuff. There’s also sexual objectification of breasts, no family community or NHS support that stops women feeding their babies.
If I had my time again, I'd have a) not had my babes by c-section (natural birth exposes babies to healthy gut bacteria) not that I had much choice here b) stuck at the breast feeding a bit longer (very middle class area here so no issues with using breasts for what they were intended..c) not given them antibiotics every time they had a minor infection that was most probably viral e.g. ears.
The Op lives in the South of France, so I am assuming these children have a healthy level of vitamin D via sunlight but prevalence of type 1 does increase the further away you get from the equator.
As has been mentioned it is a hard disease to vaccinate against given its unknown triggers. However if diagnosed early, type 1s can preserve quite a lot of their beta cell function (keto) though I suspect this is never the conventional treatment pathway...
I think prevalence is normally 1% so 5 in a year group does sound like a big exception...